Vices and clamps are well-known devices for use in holding workpieces in place on the work bed of a machine to facilitate machining or other working thereof. Such mechanical devices are not always suitable, however, for holding workpieces, especially when the pieces to be held are large or cumbersome. Metallic workpieces that are capable of magnetic attraction may also be held in place magnetically, and for workpieces that are not capable of such attraction, vacuum chucks and the like may be employed to hold them. Such vacuum holding devices typically include a means for creating an evacuation space between the workpiece or a portion thereof and the surface of the chuck or the bed on which the workpiece is to be held. Such means usually includes a sealing gasket that is provided on the chuck or work bed around or beneath the workpiece or a major portion thereof so that a partial vacuum may be generated in the evacuation space between at least a portion of the workpiece and the chuck or bed to hold the workpiece in place thereon.
When a vacuum chuck or work bed is designed to accommodate a particular workpiece of a specific size and shape, as opposed to workpieces of a variety of sizes and shapes, it is known to place the sealing gasket on the chuck or bed around the periphery of the workpiece. Thus, for example, U.S. Pat. No. 2,807,180 of Adams describes a vacuum holding chuck for a workpiece, such as a jet engine compressor wheel, that has an annular flange. The device includes an adjustable ring with a rubber sealing band on its inside surface. The ring and sealing band are placed around the outside of the annular flange of the workpiece, and a locating disk of the chuck is placed inside the flange. The cavity between the disk and the adjustable ring is evacuated to hold the flanged workpiece in place on the chuck.
U.S. Pat. No. 3,233,887 of Dunham describes a vacuum-actuated chuck that provides for accommodation of a variety of workpiece shapes by utilizing a customized chucking or surface plate that is made to conform with the shape of each particular workpiece. The chuck of Dunham includes a base having a plurality of vacuum channels surrounded by a peripheral sealing groove which contains an O-ring. The customized chucking or surface plate fits together with the base and cooperates with the vacuum channels in the base to form a vacuum chamber. In another embodiment of the method and apparatus of Dunham, the surface plate is formed of a porous material, which is masked, as by a plastic film, so as to outline the shape of the workpiece to be held. U.S. Pat. No. 3,294,393, also of Dunham, describes a similar vacuum chuck which includes a porous surface. The workpiece to be held is placed on the porous surface, and a plastic film is placed over the workpiece and the porous surface. Portions of the film are then removed from the part of the workpiece to be machined or otherwise worked. However, many cutting, milling and other working machines that are required to operate on workpieces of a variety of shapes, sizes and configurations may not readily lend themselves to the creation of a specialized sealing plate for each shape of workpiece, or to the covering of the entire workpiece with a plastic film. Vacuum chucks and other similar holding devices for such machines, therefore, have typically employed sealing gaskets that are placed beneath the workpiece on the chuck or work bed. Thus, for example, U.S. Pat. No. 2,730,370 of Brewster describes a work-holding chuck that includes a circumferential groove, or a plurality of such grooves concentrically disposed with respect to each other, and one or more air conduits through the chuck body within the circle bounded by each groove. On the side of each groove nearest the center of the chuck is a lip of particular shape that acts to retain an O-ring that is placed in the groove. The O-ring is selected to be of a size that will be retained by the circumferential groove into which it is placed in such fashion that a portion of it will extend above the chuck surface. When a workpiece is placed on the chuck body, air is withdrawn through the air conduits to pull the workpiece towards the chuck surface, thereby compressing the O-ring or -rings to seal the vacuum holding configuration underneath the workpiece and within the compressed O-ring or -rings.
U.S. Pat. No. 2,782,574 of Copold describes a vacuum work holder comprised of a pair of plates that are joined together. On the upper surface of the upper plate are arranged a plurality of circular grooves or slots, into each of which is positioned a resilient or elastomeric ring. A vacuum port inside each of the circular grooves extends through the upper plate. The lower plate is attached to the upper plate with a gasket disposed therebetween, and a vacuum pump is connected to the side of the assembly. When a planar workpiece to be held is placed on the upper plate, the vacuum ports outside the workpiece are closed off, and the vacuum pump is activated to evacuate the circular evacuation spaces beneath the workpiece and hold the workpiece on the upper plate.
U.S. Pat. No. 4,856,766 of Huberts describes another vacuum apparatus for holding workpieces such as silicon wafers or optically-readable disks. The apparatus consists of a work face having a plurality of elastic supports such as may be obtained if the work face is covered by the type of material that is used to cover table tennis paddles. A sealing gasket surrounds the work face, and a vacuum duct is provided which is in communication with the space between the elastic supports on the work face. The gasket includes an axially-movable lip that deforms against the lower surface of a workpiece that is placed on the work face when the vacuum space under the workpiece and between the elastic supports is evacuated.
U.S. Pat. No. 3,652,075 of Thompson is directed to a vacuum chuck having a work face which is provided with a plurality of parallel and intersecting grooves that form a rectangular grid pattern. The grooves are designed to receive an elongated, closed-cell rubber seal, which is discontinuous so that it may be placed in various different grooves to define a continuous seal in the work face of the chuck and under the workpiece, with the area enclosed by the seal being dependent on the size of the workpiece to be held by the chuck. Passages are provided in the chuck to communicate with the grooves in the work face. Those which are in communication with grooves outside the seal around the workpiece are closed off, while those inside the seal are left open for transmission of a vacuum to the area enclosed by the seal under the workpiece. Although the method of Thompson can accommodate workpieces of varying sizes and shapes, it requires that the work bed or chuck be prepared for each different shape or size of workpiece before the workpiece can be worked thereon. For large workpieces and those of intricate shape, preparation of the bed or chuck can take a considerable amount of time. During this time of preparation, the work bed or chuck cannot be used for its intended purpose, and is therefore unproductive. Because of this requirement for considerable set-up time for the work bed before each different size and shape of workpiece can be accommodated, the method of Thompson may not be suitable where varying sizes or shapes of workpieces may frequently be encountered. In addition, although the parallel and intersecting grooves in the bed of Thompson permit the establishment of a vacuum seal between the bed and the workpiece that extends near to the periphery of the workpiece, this method does not permit the vacuum holding force to be established at the periphery of the workpiece.
U.S. Pat. No. 5,141,212 of Beeding describes a vacuum chuck for supporting a workpiece during cutting thereof by a machine tool. The chuck includes a support plate that is provided with a plurality of holes which are in communication with one or more vacuum plenums in a base therebelow. The plenums can be selectively connected to or isolated from the vacuum source, depending on the size of the workpiece to be cut, so as to evacuate the plenum or plenums under the workpiece, but not those outside its boundary. A sheet of material such as particle board, also provided with a plurality of holes, is positioned atop the support plate, with the holes in the support plate and the particle board in alignment. A sheet of open cellfoam is laid atop the particle board and the workpiece to be cut is placed atop the cellfoam. vacuum is applied, which is channeled from the vacuum plenum or plenums under the workpiece, through the holes in the support plate and the particle board, and through the cellfoam to the workpiece. The foam does not seal against the workpiece, but as the plenum or plenums under the workpiece are evacuated, the foam compresses and its coefficient of friction increases so that the workpiece is unable to slide thereon. It would seem that the method and apparatus of Beeding would not permit the establishment of a strong vacuum force to hold the workpiece in place, since no seal is provided between the workpiece and the vacuum source. However, many metalworking and other operations require that a strong holding force be established. Some such operations also require that high-powered suction nozzles be employed to remove cuttings or chips that are generated during working, and such nozzles may generate such powerful suction forces, especially at the edges of relatively thin workpieces, that the vacuum forces holding the workpieces may be overcome, and the workpieces may be dislodged from the chuck or work bed. Even those methods that may be capable of establishing a seal near the periphery of a workpiece, such as that of Thompson, may not be able to provide a seal sufficient to hold against strong suction forces. Therefore, it is frequently important that a vacuum holding method and apparatus operate by establishing a vacuum holding force against the workpiece even to the outer edges or periphery. However, as has been seen, known methods for establishing a vacuum holding force around the periphery of a workpiece either require that a customized work surface or sealing plate be provided for each size and shape of workpiece, or that the entire workpiece be covered by a sealing film. In addition, as has been illustrated, most of the known vacuum chucks, including those that operate by establishing a vacuum holding force against only a portion of the workpiece, are provided with precisely located and machined grooves or other features of the work bed. These features may be expensive to provide, and they may be subject to damage with repeated use, especially where large or massive workpieces are involved.
In the production of aircraft wing panels, for, example, it is necessary to hold large workpieces of metal plate, such as aluminum alloy planks, on the work bed of a milling machine that is used to form the planks into the desired shapes. Such workpieces may range from 150-1260 inches long by 40-130 inches wide, and they may weigh as much as 15000 pounds. Consequently, these workpieces are typically quite large and cumbersome, but they can vary considerably in shape and size.
In the operation of such a milling machine, the workpiece is held on a work bed of fixed height, and a milling cutter, making successive passes across the workpiece, mills the plank into the general configuration of an aircraft wing panel. Despite the large size of the workpieces, the milling machine may be utilized to mill the peripheral edges of the wing panels down to a thickness of less than one-half inch. As the milling or cutting operation is carried out, a powerful suction device, which is carried by the milling cutter head, operates to remove the cuttings or debris produced by the milling operation. The milling machine with which the inventor is most familiar, an Ingersol Precision Milling Machine manufactured by the Ingersol Corporation of Rockford, Ill., uses a vacuum holding method that is somewhat similar to the method of Thompson described herein to hold the workpieces on the work bed. Such method requires that a series of intersecting grooves be provided in the work bed, which are in fluid communication with a series of vacuum ports that extend through the bed. Prior to the milling of a wing plank of a particular size and shape, considerable time is required to prepare the bed for use by placing one or more elongated, closed-cell rubber seals into the various different grooves beneath the workpiece and as close to its peripheral edges as the orientation of the intersecting grooves permits. Despite these efforts, however, the suction device carried by the milling cutter head sometimes overcomes the vacuum holding force that is established between the workpiece and the bed, and it pulls the workpiece from the bed. Furthermore, the extensive time required to prepare the bed for each wing panel leaves the bed unproductive for its intended use for hours at a time.
It would be desirable, therefore, if a method could be provided for establishing a strong vacuum holding force between a workpiece and a work bed which extends to the periphery of the workpiece, regardless of the size or the shape of the workpiece. It would also be desirable if a method could be provided for establishing a seal between a workpiece and a work bed that would permit the provision of such vacuum holding force without requiring that any preparatory operations be performed on the bed. It would also be desirable if a method could be provided for establishing a strong force to hold a workpiece on a work bed without requiring a precise and complicated arrangement of grooves and/or other surface features on the work bed.